To dewater sludge such as excess sludge discharged from biological treatment process of organic waste water including sewage water, coagulation-flocculation treatment is conducted by adding a polymer flocculant into the sludge to form flocs and dewatering treatment is then conducted. In the coagulation-flocculation treatment, in order to form tough flocs so as to improve dewatering efficiency, an amphoteric polymer flocculant may be added after an inorganic coagulant such as polyferric sulphate is added to neutralize charge of the sludge.
FIG. 7a is a schematic sectional view showing a conventional flocculation reaction apparatus for flocculation treatment by adding a polymer flocculant to sludge (raw sludge). The raw sludge is introduced by a sludge pump P into a flocculation reaction tank 1 where the polymer flocculant is added to the raw sludge. The polymer flocculant and the raw sludge are agitated and mixed by an agitator 1M and the flocculated sludge is discharged. The polymer flocculant may be injected at an introducing piping of the raw sludge.
FIG. 7b is a schematic sectional view showing a conventional coagulation-flocculation reaction apparatus for coagulation-flocculation treatment by adding an inorganic coagulant and an amphoteric polymer flocculant to raw sludge. The raw sludge is introduced by a pump P into a first reaction tank 2 where the inorganic coagulant is added to the raw sludge. The inorganic coagulant and the raw sludge are agitated and mixed by a mixer 2M and the mixture is then introduced into a second reaction tank 3 where the amphoteric polymer flocculant is added to the mixture. The amphoteric polymer flocculant and the mixture are agitated and mixed by a mixer 3M and the flocculated sludge is discharged.
In the flocculation treatment by adding a polymer flocculant to sludge, slow agitation is required because flocs are broken into small flocs by strong agitation. However, since liquid solution of the polymer flocculant has high viscosity, it is difficult to uniformly diffuse the flocculant into the sludge by the slow agitation. In case of employing the slow agitation for the purpose of preventing the flocs from being broken into small flocs, it takes a long time to diffuse the polymer flocculant and it causes irregularity in adhesion of the polymer flocculant on the surface of sludge so that sufficient flocculation function can not be obtained. As a result, it is required to increase the adding amount of the polymer flocculant. Though rapid agitation is preferable for uniformly diffuse the polymer flocculant, the rapid agitation makes flocs into small flocs so that it is impossible to grow flocs.
Strong agitation is required also for achieving effective reaction of the sludge with the inorganic coagulant. In this case, unlike the flocculation treatment by the polymer flocculant, it does not matter if flocs are broken into small flocs. However, too much power is consumed for the agitation. Further, in case of raw sludge containing foreign substance such as mixed crude sludge of sewage water, agitation blades may be clung thereto with foreign substances so that smooth agitation may be disturbed.
Japanese Utility Model Publication H08-961B discloses a flocculation reaction apparatus which is provided with a line mixer for promoting reaction of sludge with a polymer flocculant. In the flocculation reaction apparatus, agitation blades of the line mixer is clung thereto with foreign substances in the sludge so that piping may be clogged with time.
Normally, piping is designed to have a large diameter not to increase pressure loss in order to lower the discharge pressure of a pump. In such piping, aqueous suspension flows in laminar flow so that agitation effect can not be obtained.
Japanese Patent No. 2965582 discloses a flocculation reaction apparatus in which a reduced-diameter portion is formed on the way of sludge piping, thereby achieving uniform diffusion of flocculant by turbulent flow in the reduced-diameter portion. However, uniform diffusion of flocculant by forming the reduced-diameter portion on the sludge piping is insufficient. In case of using a polymer flocculant, a certain amount of processing time is required to form flocs. Enough processing time can not be taken only by forming the reduced-diameter portion on the way of the sludge piping. Therefore, preferable flocs are not formed.